This invention relates to a method of using rastophotogrammetry and Placido-disc video keratoscopy in ophthalmological surgery, more specifically, to a method of using rastophotogrammetry and Placido-disc video keratoscopy to evaluate a laser used in refractive surgery so as to measure the amount of unwanted lens effect delivered by an excimer laser or other ultraviolet or infrared laser beams.
Rastophotogrammetry has been used in ophthalmological surgery to measure the surface contour of the optic nerve head and to measure corneal surface curvatures. In the technique of rastophotogrammetry a series of parallel lines or a grid is projected on the surface to measured. Computerized digital analysis of a video image is performed to detect elevations or depressions of the surface being measured. Rastostereographic imaging is combined with image processing computer software to produce a model of the topography of the cornea, for example.
Prior art uses of the technique have been limited to measuring epithelialized corneal surfaces, before and after refractive procedures such as radialkeratotomy and excimer photo-refractive keratoscope. Although useful, prior art applications of the technique only measure epithelialized corneal topography before and after surgical intervention. There is no predictive value in this technique, that is, the rastophotogram only measures changes in the corneal topography retrospectively. If the laser used during surgery is not properly calibrated, the laser may have unwanted lens effect by removing corneal tissue in an uneven pattern leaving depressions (hot spots) or elevations (cold spots). The goal of the surgery is to perform a uniform ablation with a uniform laser beam, i.e,. a laser beam having no unwanted lens effect. Furthermore, the prior art uses of rastophotogrammetry do not measure topographical changes in optic implants or on contact lenses.
Recently it has been determined that Placido-disc videokeratoscopy can be used to determine surface contour, particularly to visualize and determine the surface contour of an "Contact lens" or artificial cornea. The Placido-disc video keratoscope is a type of computerized videokeratography now available to ohthalmological surgeons. The instrument allows the surgeon to measure and modify corneal curvature. The basic videokeratograph instrument includes a light source projected onto the cornea. The modification of the light by the cornea is captured by a video camera and the information is analyzed by computer software. The data is displayed in a variety of formats including photographs and on a screen.
The Placido-disc imaging is an extension of the single mire used in the keratomoter. A series of rings is projected onto the cornea, and the reflected images are detected by a video camera. Curvature data is derived from the measured distances between the rings. The patient is placed before a corneascope projecting a 16-ring conical Placido-disc. The scope is positioned in front of the cornea. The instrument contains a video camera for image capture. The computer digitalizes or converts the data obtained from the video output into a form that can be analyzed. A number of highly sophisticated programs convert the data into a series of color graphics displays. Hard copies can be obtained from a color printer or a camera. The color graphics provide topographic maps. Systems currently commercially available include the EyeSys Corneal Analysis System (CAS) (EyeSys Laboratories) and the Topographic Modeling System (TMS) (Computed Anatomy, Inc). Although Placido-disc videokeratoscopy systems work poorly on deepithelialized cornea after radial keratotomy, they can be used to evaluate the effect of laser ablation on an artificial cornea.